In the last two decades, stable carbene chemistry has been the focus of extensive research.[1] This is largely due to the fact that NHCs, such as the imidazolylidenes 1, have found utility as ligands for catalysts[1b, 1f, 2] and the stabilization of reactive species,[3] as well as catalysts in their own right[4] (FIG. 1). Typically, NHCs 1 are generated from imidazolium cations by deprotonation of the most acidic proton in the C-2 position of the aromatic ring with a strong Brønsted base. In 2001, Crabtree and coworkers[5] made a key discovery that it was possible for imidazolylidenes to adopt a C-5 coordination mode with transition metals. Evidence has since mounted that these “abnormal” NHCs[6] convey distinct and sometimes superior catalytic properties[6f, 6g] to the metals they bind, compared to the normal C-2 isomer. In 2009, Bertrand and coworkers reported that if the acidic proton in the C-2 position of the imidazolium salt precursor was substituted by a hydrocarbon fragment, deprotonation at C-2 could be blocked and abnormal C-5 imidazolylidenes 2 could be isolated as metal free species (FIG. 1).[7] In 2010, Robinson and coworkers reported[8a] that imidazolium salts could be deprotonated at both the C-2 and C-5 positions to afford anionic species 3 that can bind to two metal fragments (FIG. 1).[8]
Another class of molecules that contain an unusual form of carbon, wherein the carbon atom forms 6 chemical bonds with neighboring elements, are icosahedral carboranes.[9] In contrast to carbenes, icosahedral carboranes are extraordinarily stable molecules and certain members of these families, such as the carba-closo-dodecaborate anion HCB11H11− 4, reported by Knoth in 1967,[10] are legendary for their inert properties (FIG. 1). The carborane anion 4 delocalizes its charge throughout the 12 cage atoms, rendering the cluster and its derivatives very weakly coordinating. The combination of weak coordinative ability and resistance to chemical decomposition explains why these molecules are widely used as counteranions for highly reactive cationic species.[9a, 11]
Over the last decade, numerous technological advances in rechargeable portable devices and electric vehicles have been made. However, innovations that reduce the cost, improve the sustainability, and increase the storage capacity offered by state-of-the-art lithium ion technology has not kept pace with this revolution.[20] The need for advances in battery technology is particularly urgent for the development of practical electric automobiles that can travel significantly further than 300 miles per charge[20] Magnesium-based batteries[21] are attractive energy technologies that have the potential to disrupt the current predominance of lithium ion batteries in the marketplace. In contrast to Li, Mg is less expensive, much more abundant (4% of the earth's crust), more tolerant of air, and does not form hazardous dendrites. The absence of dendrite formation during Mg deposition allows the utilization of pure Mg anodes, which drastically increases the energy storage capacity of the battery. In addition, since Mg is a small divalent atom it can store twice the amount of electrons and thus more energy than Li (theoretical volumetric capacity of metallic Mg=3832 mA h cm−3; Li=2062 mA h cm−3). However, a key barrier to the development of practical high capacity Mg batteries, is that suitable electolytes are elusive.[21] Electrolytes for Mg batteries must be completely resistant to decomposition at the prefered voltage windows (1-5 v vs Mg0/+2). The present invention provides new materials to meet these needs.